1. Field of the Invention
This invention relates to a method of producing a semiconductor integrated circuit, and more particularly to a method of producing a complementary MOS (Metal-Oxide-Semiconductor) type integrated circuit (hereinafter referred to as a "CMOS type IC" having a parasitic channel stopper region which is self-aligned with both a well region and a field oxide film.
2. Description of the Prior Art
As a method of forming a well region and a parasitic channel stopper region in a CMOS type IC, one such as that shown in FIGS. 1a and 1b has heretofore been known.
This conventional method will be explained below.
Referring first to FIG. 1a, a P-type well region 2 is formed in a surface region of an N-type silicon substrate 1, and a silicon nitride film 4 is deposited on the upper surface of the substrate 1 through a thin silicon oxide film 3 in such a manner as to cover the well region 2. Then, a photoresist layer 5 is formed on the silicon nitride film 4, the layer 5 having an opening corresponding to a desired parasitic channel stopper pattern, and with this photoresist layer 5 used as a mask, the silicon nitride film 4 is selectively etched to provide a hole 4a which corresponds to the parasitic channel stopper pattern. Thereafter, boron ions are selectively implanted into the surface of the substrate 1 using the photoresist layer 5 and the silicon nitride film 4 as a mask and through the silicon oxide film 3.
Then, as shown in FIG. 1b, the silicon nitride film 4 is provided with a hole 4A for selective oxidation by a photolithography technique, and with this film 4 used as a mask, selective oxidation is carried out to form a field oxide film 6, together with a P.sup.+ -type parasitic channel stopper region 7 which is located directly below the film 6, the region 7 containing the boron which has been implanted in advance.
According to the above-described conventional method, the hold 4a is formed in the step shown in FIG. 1a using a photomask in such a manner that the hole 4a is aligned with the P-type well region 2. In consequence, boron ions may be undesirably implanted into a portion which extends from the end edge of the region 2 by a width W due to a possible error in the alignment using the mask. As a result, the P.sup.+ -type parasitic channel stopper region 7 may be formed such as to extend outward from the P-type well region 2 as shown in FIG. 1b. This leads to a reduction in the packing density, unfavorably.
In addition, the step shown in FIG. 1b needs a photolithography step including alignment using a photomask for forming the hole 4A, which means that a total of three photolithography steps are needed to obtain the structure shown in FIG. 1b, including the step which is carried out when the P-type well region 2 is formed. It is necessary, in order to increase the packing density and reduce the production cost, to reduce the number of photolithography steps.